Germicides or antibacterial agents used to provide antibacterial efficacy to skin cleansing compositions are known in the art. Thus, for example, bars containing bacteriostats such as Triclosan (DP300) or Triclocarbon (TCC) are known.
Bacteriostats inhibit the growth of bacteria on skin when they are deposited on the skin. Thus, everything else being equal, the extent of antibacterial activity in any soap bar has until now (measured by long-term substantive effort) been dependent on the nature and amount of antibacterial in the soap bar.
The efficacy of anti-bacterial activity of skin cleansing products containing bacteriostats, however, can be measured in a variety of ways.
The efficacy of anti-bacterial activity of skin cleansing products containing bacteriostat, for example, are generally measured in two types of assays. The first type measures the effect of anti-bacterial agents deposited on skin and is thus reflective of substantively effects. The second type measures the ability of the formulation to cause quick kill (less than 1 minute) of bacteria as determine by in-vitro solution tests.
The time of contact of bacteria with the cleanser in the in-vitro, short time kill assay is somewhat reflective of cursory wash conditions. In fact, cursory wash may take much less than one minute.
Since many or most people who wash with soap bars may not use the bar for longer than a few seconds (average wash time for children may be 10 seconds or less), it becomes apparent that there is a need to deliver anti-bacterial activity in a short period of time (e.g., 60 seconds or less, preferably 30 seconds or less) and, therefore, ways for reassuring quick-kill effect on bacteria are quite critical.
In this regard, the applicants have unexpectedly discovered that immediate bactericidal activity, i.e., quick kill of bacteria, is a function not of the anti-bacterial agent, but rather of the soap, specifically its molecular solubility and surface activity. Thus, factors which enhance both molecular solubility and surface activity increase the anti-bacterial activity under quick kill conditions. Moreover, applicants have found that higher molecular solubility and surface activity correlates with higher zein dissolution by the soap (zein is a relatively insoluble corn protein and the ability of the soap/surfactant to solubilize zein reflects its ability to interact with proteins). That is, higher zein dissolution is essentially a reflection of the ability of the soap to interact with bacterial membrane proteins and cause its kill.
It is well known that soap bars are made with both soluble (e.g., coco soaps), and insoluble (e.g., tallow soap) components. When the soluble fraction dissolves, it can exist in the form of monomeric species in the low concentration range and form aggregates called micelles at some critical concentration defined as CMC. Thus, CMC for a pure component represents the maximum level of its monomeric species available in the aqueous phase. The aqueous solubility and the CMC of soaps and other surfactants decrease with increase in hydrocarbon chain length. The surface activity of the surfactant, on the other hand, increases with increase in chain length. Thus, solubility and surface activity show opposite trends with increase in chain length (e.g., less soluble and more surface active). For a soap molecule to interact with bacteria and cause its kill, it has to be present in the aqueous phase and bind to the membrane when exposed to bacteria. It is generally believed that monomers bind to membranes and the micelles essentially act as a reservoir of monomers. The extent of binding increases with increase in the surface activity of the molecule, provided it is available in the aqueous phase. Thus, for increasing the quick kill, at a given temperature, there should be sufficient amount of highly surface active soap molecules in a monomeric form available for interaction with the bacteria. This implies that the soap molecules should have high surface activity, high CMC and adequate solubility. For this reason at a given temperature there exists an optimum chain length which exhibits adequate solubility and surface activity to bind to the bacteria. At room temperature this optimum chain length for soaps is about C12.
For a given chain length soap, its molecular solubility can also be increased by appropriate choice of counterions. For example, it is known that the solubility of the soaps follow the order: NH4 soaps or triethanolamine soaps&gt;K soaps&gt;Na soaps. Thus, the antibacterial activity of soaps also can be expected to follow the order: NH4 soaps&gt;K soaps&gt;Na soaps. Therefore, for a given soap system, its immediate kill activity can be enhanced simply by changing the counterions from Na to K or NH4. As noted previously, another method of increasing the solubility is by simply increasing the "soluble" fraction (for a given temperature) of the soap in the bar.
The net effect of solubility and surface activity is reflected in bacterial kill as well as in zein solubilization.